1. Field of the Invention
The present invention relates to a pellicle which is bonded to a mask used in a photolithography process to protect the mask which process is a part of method for semiconductor integrated circuit fabrication.
2. Description of the Related Art
A mask is an original plate of a circuit pattern used for the transfer of a circuit pattern of a semiconductor integrated circuit onto a wafer, that is, the mask is a glass substrate having an original circuit pattern fabricated of opaque thin membrane formed thereon. To transfer the circuit pattern onto the wafer, the mask is set above a wafer, then ultra violet light, X-rays or the like are irradiated onto the mask.
When foreign matter such as dust is adhering to the mask, the exact shape of the shadow of the foreign matter is transferred onto the wafer. Subsequently, defects are made in the circuit pattern transferred onto the wafer, resulting in a lowering of the reliability of the semiconductor integrated circuit. To prevent adhesion of foreign matter like this to the mask, a structure having a transparent protective membrane, called a pellicle, is mounted to the mask surface.
FIG. 3 is a partially sectional view showing a structure of a conventional pellicle. As shown in FIG. 3, a pellicle 30 comprises a pellicle membrane 31 which is a protective membrane fabricated from a transparent member, a pellicle frame 32 to hold the pellicle membrane 31 at a predetermined distance away from a surface of a mask 34, and an adhesive 33 to fix the pellicle frame 32 to the surface of the mask 34. The pellicle membrane 31 is fabricated from a transparent thin membrane formed from nitrocellulose or the like, and prevents adhesion of foreign matter from outside onto the surface of the mask 34 by being fixed at a predetermined distance away from the surface of the mask. Even when foreign matter does adhere to the pellicle membrane 31, the shadow of the foreign matter is not transferred onto the wafer because the shadow of the foreign matter adhering to the pellicle membrane 31 is out of focus during exposure of the wafer since the pellicle membrane 31 and the mask 34 are spaced apart.
To mount this type of pellicle 30 to the mask 34, first, a side of the pellicle 30 with the adhesive 33 applied thereto and a surface of the mask 34 having an original circuit pattern formed thereon, are positioned face-to-face. Then, the pellicle 30 is press-adhered to the mask 34 by the application of a force against the mask 34 from above the pellicle 30.
However, when this type of pellicle 30 is fixed to the mask, the adhesive 33 is squeezed out due to the pressure applied to the pellicle 30, and a part of the adhesive 33 from an end surface of the pellicle frame 32 adheres to the surface of the mask on the inside and outside of the pellicle frame 32. FIG. 4 is a schematic sectional view showing the squeezed adhesive 33' adhering to the surface of the mask 34 inside the pellicle frame 32. The adhesive 33' adhering to the surface of the mask 34 inside the pellicle frame 32, as shown in FIG. 4, deteriorates due to exposure to the light from an exposure apparatus and loses its tackiness, and may move near to the original pattern of the mask due to vibration during transportation of the mask or the like. The problem has arisen that this deteriorated adhesive 33' causes defects in the mask 34 by adhering to the original pattern of the mask 34.
In order to solve this type of problem, the technology described in Japanese Patent Application Laid-Open (JP-A) No. 6-19124 provides a pellicle such as that shown in FIG. 5 (FIG. 5 shows a pellicle which is mounted to the opposite surface of the mask to that in FIG. 4). A groove 41a is formed on an end surface of a pellicle frame 41 facing a mask 43, and a pellicle is fixed to the mask 43 by an adhesive with which the groove 41a is filled. By utilizing this structure, bleeding of the adhesive onto the mask 43 inside the pellicle frame 41 is prevented.
However, in the pellicle structure described in the aforementioned JP-A-No. 6-19124, the problem has been that it is difficult to adjust the amount of the adhesive 42 to be placed in the groove 41a. For example, too small an amount of the adhesive 42 makes it difficult to sufficiently fix the pellicle on the surface of the mask 43 because the adhesive 42 is not able to fully contact the mask 43. On the other hand, too large an amount of the adhesive 42 causes problems with the adhesive 42 being pushed out all over the end surface of the pellicle frame 41 from the groove 41a upon pressing the pellicle against the mask 43, and the pushed-out adhesive 42 adheres to the mask 43 on the inside and outside of the pellicle frame 41. There has been a further problem that when the adhesive 42 is placed in the groove 41a like this, the bonding area of the mask 43 and the adhesive 42 in the state shown in FIG. 5 is too small for a sufficient bond strength to be obtained.
The aim of the present invention, therefore, is to provide a pellicle which does not cause lowering of reliability of the mask due to the adhesion of adhesive pushed out from a pellicle frame to a surface of a mask when fixing the pellicle to the mask.